Wireless networks are evolving rapidly, requiring high capacity data and video services and growing bandwidth use per subscriber.
A generic wireless system includes one or more subscriber units (SUs) that communicate with a base station (BS) via an air interface. Transmission from the base station to the subscriber units are commonly referred to as “downlink” transmissions. Transmissions from the subscriber units to the base station are commonly referred to as “uplink” transmissions.
An objective of these wireless communication systems is to provide communication channels on demand between the subscriber units and their respective base stations in order to connect a subscriber unit end user with the fixed network infrastructure.
These systems are characterized by a layered architecture including a Media Access Control (MAC) layer and a physical layer. The MAC layer manages the access of a subscriber to the system, the inter-channel handoff, the power control, privacy of traffic data, the access to the shared radio resources and the frame transmission construction. The physical layer performs data coding, modulation, transmission and reception.
A major objective of the communication services is to provide communication with a satisfactory Quality of Service (QoS). QoS is a measure used to specify the parameters that characterize the service offered by the network, namely the degree of satisfaction of the users with the communication services provided by the system. The QoS measures commonly used include call blocking and dropping probability, packet loss probability, transmission delay and delay jitter, and bit error rate.
In general a wireless network will support a variety of services with different traffic characteristics and QoS requirements: in particular, some services, such as voice, are regarded as high-QoS services, or guaranteed traffic, while other services, such as Internet browsing, video streaming and video conferencing, require lower levels of QoS. Based on the type of service and of their required QoS, various availability levels can be defined in the system, with different degrees of continuity of the connection required by the services.
Another important goal that needs to be considered in wireless networks is resource utilization. Due to limited radio spectrum, a highly utilized system that can provide a satisfactory amount of QoS to the users is always a desired solution. However, high resource utilization and QoS provisioning are always conflicting goals. Resources are set-aside for active users, so that their QoS can be maintained, but unused resources mean low utilization. In order to have a balance in the two conflicting goals, the amount of reserved resources should be calculated carefully.
With respect to these goals, a central role is played by the Connection Admission Control (CAC) module, a module in the MAC layer which is responsible for determining if a connection can be allowed given the current channel conditions.
The MAC layer, at the base station side, sends connection requests, specifying their QoS requirements and their availability level, to the CAC module that, on the basis of this information and the available system bandwidth, determines if connections can be admitted or not in the system. When a connection is admitted, the CAC allocates to it in a static way the bandwidth, called admission-bandwidth, necessary to satisfy its QoS requirements and to preserve those of already admitted connections.
In recent years, in order to improve the efficient exploitation of resources, Adaptive Modulation and Coding (AMC) techniques have been developed. The basic idea behind AMC is that the modulation and coding scheme on the communication channels is not fixed statically, but can vary dynamically over time in response to the varying quality of the radio link. More specifically, AMC denotes the matching of the modulation, coding and other signal and protocol parameters to the conditions of the radio link, such as the pathloss, the interference due signals coming from other transmitters, the sensitivity of the receiver, the available transmitter power margin, and so on. The modulation and coding scheme is varied accordingly, and consequently the bit rate and robustness of data transmission. The process of AMC is a dynamic one and the signal and protocol parameters change as the radio link conditions change.
AMC is a function performed at the MAC layer and implemented at both the transmitter and the receiver. The AMC function chooses the link modulation and channel coding among a set of predefined AMC schemes with different spectrum efficiency and robustness according to the link conditions. Most commonly the link conditions are monitored by measuring the signal-to-noise ratio (SNR). Each AMC scheme is mapped to a given range of SNR that defines the thresholds for changing from one AMC scheme to another.
Adaptive modulation systems invariably require some channel information at the transmitter: in order to support adaptive modulation and coding, the base station, that commands the uplink and downlink AMC schemes' change, needs to periodically measure uplink conditions and receives information from the subscriber stations about downlink conditions. The base station, on the basis of such information, chooses the link modulation and channel coding among a set of predefined AMC schemes, each mapped to a given range of SNR values, and successively communicates to the subscriber stations their AMC schemes.
AMC systems exhibit the intrinsic characteristic of having bandwidth variable with channel conditions and consequently they require efficient connection admission control and traffic handling mechanisms in order to bring advantages. Whenever link conditions deteriorate, since the link payload decreases, the admission control of a system that adopts the AMC technology must guarantee that services with high availability requirements, such as voice, are not affected. Services requiring lower availability requirements can instead be transmitted without many guarantees under worse link conditions.
In light of the above a major technical issue in current AMC systems is to provide suitable CAC policies able to assure guaranteed traffic and at the same time optimize bandwidth utilization.
The solutions proposed in the technical literature for Connection Admission Control in AMC systems do not completely exploit AMC system potential: most of the adopted CAC strategies are a simple extension of CAC adopted in non adaptive systems.
The most common implementations of CAC support allocation of resources to services with high QoS in a static way: the system calculates if the bandwidth corresponding to the most robust AMC scheme is available, and in that case the connection is accepted, otherwise is rejected. In other words, the CAC performs its decision regardless of link conditions, not improving the number of connections with high QoS admitted in the system when AMC is adopted with respect to traditional systems. In case of good link conditions, the additional link capacity provided by more efficient AMC schemes is only exploited by low QoS traffic.
Solutions developed during recent years, aiming at increasing the admission rate of guaranteed traffic, are based on the connection suspension concept. For instance, in U.S. Pat. No. 7,023,798, connections are admitted in the system according to a pre-planned AMC scheme more efficient than the most robust one. When resource needs increase, some connections are suspended on the basis of precedence priority levels. This solution is rather conservative, since it does not react dynamically to the link conditions and therefore does not fully exploit the current bandwidth availability.
Other solutions, which apply to point to multipoint systems, for instance U.S. patent application Ser. No. 08/910,147 (now U.S. Pat. No. 5,983,101), exploit AMC schemes differentiation not only in terms of bandwidth efficiency, but also in terms of coverage range and interference immunity to estimate the optimal distribution of AMC schemes among terminals/connections in the cell. These solutions are rather complex to implement.